There is a maximum number of networks and hosts that can be assigned unique Internet Protocol (IP) addresses using the Internet's 32-bit addressing scheme. Internet IP addresses can be assigned in “classes”, with Class A, Class B and Class C being commonly assigned classes. Each class address has two parts: a first part to identify a unique network, and a second part to identify a unique host within that network. Class A addresses support the largest number of hosts, while Class C addresses support the smallest number of hosts. Class addresses are assigned based on the number of host addresses required, but because of the coarseness of three-class system, a large number assigned host addresses can be unused.
A revised method of IP address assignment called Classless Inter-Domain Routing (CIDR) subsequently emerged. A CIDR address includes a 32-bit IP address, along with an additional descriptor that defines the number of bits of the IP address used to identify a particular network. The number of bits used in the network identifier part can vary anywhere from thirteen to twenty-seven bits, thus allowing more flexibility in the assigning of host addresses over the original class system. Nevertheless, the demand for IP addresses continued to outpace the number of assignable IP addresses. CIDR is described in detail in Fuller, V., Li, T., Yu, J., and K. Varadhan, “Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy”, RFC 1519, September 1993.
As a result, the concept of a private address space was introduced. The Internet Assigned Numbers Authority (IANA) reserved three blocks of IP addresses for use by “private” networks. The private addresses have no global meaning. That is, routers in networks not using private address space, especially those of Internet Service Providers (ISPs), are expected to be configured to reject (or filter out) routing information about private network addresses. The address space can thus be used (and reused) by many enterprises, making the IP addresses within the private address space only unique within a given enterprise. Private address space is described in Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. J., and Lear, E., “Address Allocation for Private Internets”, RFC 1918, February 1996.
The use of private network address space in managed enterprise networks presents challenges in the task of network management. First, a managed network spanning multiple private networks using private network address space can result in the same IP address being assigned to multiple network devices. This can lead to uncertainty in determining the source of network events and ambiguity in the display of devices in network topology maps. In addition, because the routers in networks not using private address space are expected to be configured to reject routing information about private network addresses, network management devices external to a private network cannot directly poll and collect data from operating within the private network.